One of the simplest and most positive tests is to turn the BPL service on and
off and observe changes in interference, thus identifying if BPL is the
source.
But to my knowledge no BPL test site has done such tests.

I have done just that in Emmaus, PA and in Potomac, MD. The correlation
between the noise heard and the download of files by users was perfect. NTIA
did testing in the same two cities, but working with the utility companies to
do just that -- turn the system on and off. And even after working with NTIA
in Potomac, a PPL representative had the gall to tell a local newspaper
reporter that Carl and I had heard "neon signs" in the residential
neighborhood.

What about harmonics? The BPL signals on the video sounded like they had
pretty fast risetimes.

What sounds fast on a receiver will generate modulation sidebands up and down
the band, but not harmonics. I have heard harmonics in the trail areas,
perhaps down about 30-40 dB. Our 1500-watt stations can have 50 milliwatt
harmonics, so we may not want to point our fingers in that direction. :-)

Because different systems are being tried in different locations, proponents
of each system will probably proclaim the *their* system doesn't have that
problem - what you heard at the Podunk site is Brand X BPL.

Yup, been tried. Ironically, the least cooperative BPL manufacturer's system
was actually the most benign. Had they worked with me instead of turning it off
when they knew I was coming, their marketing folks could have made hay.

73,
Ed Hare, W1RFI